The conventional hot air heating system, as used in a residence or commercial establishment, includes a gas or oil fired furnace which is controlled by a room thermostat. When the thermostat calls for heat, the furnace is operated, and when the plenum temperature reaches a preset value of about 200.degree. F, the blower is operated to deliver the heated air through the duct system to the area or zone to be heated.
The conventional hot air heating system operates from a cold condition, meaning that the plenum and ducts are cold when the furnace is started up, with the result that the plenum must be initially heated before the blower is operated to deliver heat to the zone of the building, and a substantial amount of heat is lost due to the heating of the duct system.
In addition, there is substantial heat loss through the chimney in the conventional hot air heating system, due to stack heat being dissipated to the atmosphere and by virtue of natural convection of room heat through the chimney when the furnace is not operating.
Furthermore, the heating unit, as used in the conventional hot air heating system, operates at full capacity whenever the room thermostat calls for heat. This results in the room temperature going above the set temperature, and when the heating unit is de-energized, the room temperature falls beneath the set temperature before the heating unit is again energized. The result is that the room temperature oscillates or hunts about the temperature setting.
Condition response controllers, as described in U.S. Pat. No. 3,509,322, have been utilized to control the "on" time of the heating unit in accordance with the existing temperature condition. In a system such as that, the heating unit is energized at substantially full capacity when the room temperature is substantially below the desired temperature level and as the room temperature approaches the set point, the energization of the heating unit is reduced.
Periodic timing devices have also been used in conjunction with electric resistance heating units for heating food products, as disclosed in U.S. Pat. No. 3,666,921. In devices of that type, pulsations of heat are produced, and the duration of the heating pulse is controlled by the timer so that a predetermined amount of heat can be precisely programmed.